Question

This is not my ‘production code’ but a simplication of the problem for illustration purposes. Also, the title of this question is misleading because it brings to mind the ~@ expansion, which I understand, and which may not necessarily be the problem. Please suggest a better question title if you can.

Given a macro with the following form:

(defmacro my-add [x & ys] `(+ ~x ~@ys))

Now let’s say we have a list:

(def my-lst '(2 3))

Now I want a function that uses my-add that I can pass my-lst to as an arg, i.e.

(call-my-add 1 my-lst)

I define the function in what would seem to be the obvious way:

(defn call-my-add [x ys]
    (apply my-add (cons x ys)))

But:

java.lang.Exception: Can't take value of a macro: #'user/call-my-add (repl-1:60)

I’ve tried all sorts of wild tricks to get the call-my-add function to work using evals, applies, and even defining call-my-add as a macro, but they all give similar ClassCastExceptions.

Is there any way out of this?

Answer 1

No. Macros do not, cannot, will never, have access to the actual run-time values contained in their arguments, so cannot splice them into the expansion. All they get is the symbol(s) you pass to them, in this case my-list. The “way around this” is to define my-add as a function, and then (optionally) have a macro that calls that function in order to generate its code.

I wrote a blog post about this semi-recently that you might find enlightening.

You could do it with evals if you wanted to, but that is a terrible idea in almost every case:

(let [my-list '(1 2)]
  (eval `(my-add 5 ~@my-list)))